Genericons are vector icons embedded in a webfont designed to be clean and simple keeping with a generic aesthetic.

A full list of the Genericons icons is available at http://genericons.com/ but also on the WP Admin -> Apperance -> Genericon'd page.

To use any of the Genericons icons on your WordPress site you can use basic HTML (for inserting in themes and functions) or shortcodes (for use in posts or widgets). You can adjust the size of the icons via css or, when using the shortcode, the size attribute. Default size is 16px.